Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-12-11
Astronomy and Astrophysics
Astrophysics
6 pages, 5 postscript figures, proceedings of contributed talk at the St. Luc conference ``CNO in the Universe'', eds. C. Char
Scientific paper
This study calls attention to the importance of properly coupling the molecular opacities to the actual surface abundances of TP-AGB stars that experience the third dredge-up and/or hot-bottom burning, i.e. with surface abundances of carbon and oxygen varying with time. New TP-AGB calculations with variable opacities -- replacing the usually adopted solar-scaled opacity tables -- have proven to reproduce, for the first time, basic observables of carbon stars, like their effective temperatures, C/O ratios, and near-infrared colours. Moreover, it turns out that the effect of envelope cooling -- due to the increase in molecular opacities -- may cause other important effects, namely: i) shortening of the C-star phase; ii) possible reduction or shut-down of the third dredge-up in low-mass carbon stars; and iii) weakening or even extinction of hot-bottom burning in intermediate-mass stars.
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